This disclosure relates generally to heat exchangers. More particularly, this disclosure relates to improved structures and geometries for glass heat exchangers, which are more efficient in gas heat exchange.
Heat exchangers are devices that facilitate the transfer of heat between mediums. Such devices are found in a large number of applications, ranging from air-conditioning units, to engines, and so on. In some heat exchangers, efficiency is determined by the effectiveness of the heat exchanger in thermally isolating opposing sides of the heat exchanger such that a gas or other working fluid flowing therebetween transfers heat to the heat exchanger between a hot end and a cold end of the heat exchanger. One particular application of a heat exchanger where such an efficient heat gradient is of particular importance is in a cryogenic cooler (“cryocooler”), which may utilize the cold end to effectively cool various components, such as electronics, superconducting magnets, optical systems, or so on.
The primary use of the heat exchanger in systems such as cryocoolers may be to pre-cool the working gas as it is transferred from the hot end to the cold end of the machine. Such heat exchangers may be characterized by how the gas flows through the exchanger and the surrounding system. For example, many closed cycle, linear cryocooler systems utilize the Stirling cycle, wherein a working gas cyclically flows in opposing directions through the heat exchanger. Such systems are typically referred to as regenerative heat exchangers, or regenerators. In other systems, a working gas steadily flows through the heat exchanger, utilizing processes such as the Joule-Thompson effect to create the cold end. The heat exchangers of these steady flow systems are typically referred to as recuperative heat exchangers, or recuperators.
The effectiveness of heat exchangers may be dependant upon various factors, such as heat transfer effectiveness, pressure drop, heat capacity, and parasitic conduction of heat. In regenerative systems, the gas is compressed at the hot end of the regenerator, and will be allowed to expand after it reaches the cold end. The structure of the heat exchanger itself may prevent the transfer of significant amounts of heat to the cold end as it flows. In regenerative systems, the oscillating rate of gas flow is typically of a high frequency. Therefore, the rate of heat transfer from the working gas to the regenerator should be rapid to ensure a desirable amount of pre-cooling of the gas through the heat exchanger.
Minimizing pressure drop across the heat exchanger is also desirable in increasing cooler efficiency, however this is typically at odds with maximizing the rate of heat transfer because obtaining maximum heat transfer effectiveness is generally through maximizing the mount of solid surface area over or around which the gas flows, which may create flow friction for the gas, and thus increase the pressure drop. In many heat exchangers, the cross-sectional flow area and parameters of porosity for the heat exchanger are varied to balance minimal pressure drop and maximum heat transfer.
The heat capacity of the heat exchanger must be such that the exchanger may absorb heat from the working gas without experiencing an intrinsic temperature increase which may reduce system efficiency. An interplay between the specific heat of the heat exchanger materials and the specific heat of the working gas exists, and may be particularly troublesome when cryogenic temperatures are sought to be achieved at the cold end of the exchanger. As one example, the specific heat of helium (a common working gas) is relatively high at cryogenic temperatures, while the specific heat of common heat exchanger materials is lower at cryogenic temperatures than at room temperature. This may call for an increased volume or mass for the heat exchanger.
The material selection for the heat exchanger is also important in preventing parasitic conduction of heat, for example along the axis of the heat exchanger. Where a large temperature gradient occurs along the length of the heat exchanger, it is very desirable that the exchanger have low thermal conductivity along its length, as high conductivity may result in heat being conducted from the hot end to the cold end. This conducted heat is a parasitic reduction of efficiency, because it must be carried as part of the refrigeration that is produced by the cycle.
One type of conventional heat exchanger typically contains a large number of woven-wire screens (i.e. on the order of 1000 screens in some embodiments) that are packed together into a volume. The working gas flows through the screens of the volume, so that the screens, which are typically formed from stainless steel, absorb the heat from the gas. The screen material may be similar to that of typical filter screens, with hundreds of wires per inch of material and wire diameters on the scale of a thousandth of an inch. The wires are generally drawn from stainless steel stock, a material that exhibits acceptable heat capacity and thermal conductivity.
There are limitations to stacked screen heat exchangers, however. For example, the heat capacity of the stainless material drops to unacceptably low levels at low cryogenic temperatures (i.e. below 30K). Additionally, construction limitations on the screens permit only a relatively small range of regenerator porosities, the ratio of regenerator open volume to overall regenerator volume (typically 60-75%). Similarly, the pore size between rows of wire is limited. Restrictions on achievable porosity and pore size limit the ability of a cryocooler designer to effectively optimize the relationship between pressure drop, heat transfer effectiveness and heat capacity. As an example, at very low temperatures, such as those encountered in the 2nd stage of a multi-stage cryocooler, the ideal screen regenerator might have a porosity significantly lower than 60% such that the solid volume (and hence heat capacity) is increased in order to combat the reduction in specific heat of the stainless steel at such low temperatures. However, porosities significantly below 60% are difficult to obtain using stainless steel screen technology.
Another type of conventional heat exchangers contains packed sphere beds. The working glass flows through the spaces between the spheres of the exchanger, transferring heat into the spheres as it moves through the heat exchanger. The sphere bed heat exchangers have an advantage of being able to utilize materials that may not easily be formed into woven screens, such as lead or rare-earth metals, that may exhibit high specific heats at low cryogenic temperatures. Sphere bed heat exchangers also have an additional benefit of permitting a lower porosity for the heat exchanger (i.e. below 40% for some embodiments), which can be achieved due to the inherent geometry of the sphere pack. The lower porosity allows more solid material, and thus greater heat capacity, while maintaining an acceptable tolerance of pressure drop for many applications. In some cryocoolers utilizing packed spheres, temperatures as low as 11K at the cold end have been achieved. Despite this success, sphere beds are less effective at higher temperatures, where heat capacity is less of a concern than pressure drop.
A more recent development in heat exchanger technology has been the use of glass as the heat exchanging element. Glass manufacturing processes include etching, grinding, or machining, which may permit, among other things, greater degrees of shaping and control of the porosity of the heat exchanger. The present manufacturing of heat exchangers typically involves etching or scoring panes of glass, which are then bonded together to form heat exchange elements. Among other things, the bonding process, or the presence of the bond between the glass layers, may reduce the effectiveness of the glass in exchanging heat with the gas flowing through the etched layers. In other cases, heat exchangers may be formed by a plurality of perforated glass plates, having slots etched in each layer, separated by spacers.
What is needed is, among other things, improvements over known heat exchanger geometries and structures, which permit a more effective heat transfer without resulting in an excessive pressure drop.